In molding rubber articles such as tires, the initial heating of the tire surface in contact with the hot mold rubber so that under the influence of internal molding pressure local rubber flow occurs. Venting is required to allow pockets of air which may become trapped between the green tire carcass and the hot mold to escape so that every part of the curing tire surface contacts the mold and the tire is thus vulcanized with a perfect impression of the mold detail.
Variously configured mold vents have thus been proposed and used to accomplish the above-stated objective. One approach is in the use of small diameter holes drilled through a mold wall normal to the interior surface. Another common approach is the use of “insert vents” that comprise small bore tubes introduced through the mold wall. Once trapped air has vented through the hole rubber begins to flow through the vent. However, the small diameter of the hole ensures that the rubber cures rapidly thus plugging the vent hole and sealing the mold. After completion of the tire curing process these plugs of rubber that are still attached to the tire surface are pulled out of the vent holes when the tire is extracted from the mold.
Such plugs of rubber detract from the visual appeal of the molded tire and are usually removed by trimming. A trimming operation, however, is time consuming and undesirably adds to the cost of the finished tire.
Another problem frequently encountered is that such plugs of rubber may break off when the tire is being extracted from the mold, thus blocking the vent hole. A blocked vent hole may not be immediately apparent and can cause subsequent poor quality moldings.
To solve the aforementioned problems, “spueless” vents have been proposed that allow trapped air to escape but which close to prevent rubber flow. U.S. Pat. Nos. 4,492,554 and 4,347,212 disclose examples of such “sprueless” vents. These known vents comprise a valve held normally open by a coil spring to allow the passage of air. The valve is closed by flowing rubber that moves a valve head portion against the spring tension into seated engagement with a valve seat.
While working well, certain problems remain unsolved by such existing vent configurations. One problem that reoccurs with existing vent configurations is that the vent sleeve is prone to becoming dislodged over time from the mold wall to which it attaches. In other instances, a problem may exist when molds containing such vents are cleaned. Cleaning is commonly effected by blasting the mold surface with a mildly abrasive material such as a plastic grit. However, because conventional vents are open during the cleaning operation it is possible for the cleaning medium to enter and block the vent.
United Kingdom published application GB 2,339,163 A discloses a further embodiment of a known vent that seeks to overcome the problem that occurs when a cleaning medium penetrates into the vent. The application discloses a vent configuration in which a first closure means (bias spring) is used in combination with a second closure means to close the vent when it is cold. This mechanical closure system is an improvement over the prior art but is relative expensive and complicated to fabricate. Moreover, the vent construction does not prevent the vent sleeve from becoming dislodged over time from the mold wall.
Accordingly, the industry remains in need of a vent plug system that can function effectively in venting air when open yet avoid vent obstruction when subjected to a cleaning procedure. Still further, a suitable vent system will be relatively simple and inexpensive to manufacture, incorporate and utilize within a mold, and replace if necessary.